A lot of C14 riders, myself included, switch out their 190/50 rear to a 190/55 for better handling. Ninja 1000's too. Not too many purposely go the other way, other than maybe for those inseam challenged wanting a bit lower seat height.
The slightly lower rear ride height means your front rake will be a fraction less steep and the trail a bit longer. That combined with the 50 series profile means your turn-in might be a tiny bit slower, but you probably won't notice it.
Did they ask you first before installing it?
2014 K1600GT Sport